Shape memory plastics can have a permanent original shape stored in addition to a visible temporary shape. By activating an external stimulus, for example a temperature increase, the permanent shape can be almost completely recalled, i.e., the plastic “remembers” its original shape.
The material is thereby first transformed into its original, permanent shape by using conventional processing methods, such as extrusion, injection molding as well as also from a polymer solution. The plastic is then deformed and fixed in the desired temporary shape. This process is called programming. It includes either heating the sample above the switching temperature Tsw and below the highest transition temperature Tperm, deforming the sample by forcing a shape and then cooling the sample while maintaining the forced shape. Alternatively, programming can also be performed by deformation at a low temperature below the switching temperature Tsw (“cold stretching”).
The permanent shape is now stored, while the temporary shape is actually present. The memory shape effect is activated by heating the plastic to a temperature higher than the switching temperature Tsw, with the material returning to the stored permanent shape due to its entropy elasticity.
If the transition from the temporary to the permanent shape is caused by a temperature change, then this is called a thermally induced shape memory effect. This thermally induced shape memory effect is in most situations realized by direct temperature action (application of heat). In addition, indirect heating is known. For example, it is known to incorporate nanoscale particles in a shape memory plastic, which to interact with an externally generated alternating electromagnetic field (EMF). In this way, a temperature rise in the shape memory polymer can be obtained “indirectly” by way of the embedded nanoparticles.
The ability of these nanoparticles to realize heating through excitation via an EMF has already been implemented in several applications. The electric eddy currents induced in electrically conducting particles in the EMF generate heat. This principle was not only used in technological applications, for example as innovative gluing technique for hardening adhesives (WO 2004/056156 A, WO 2002/012409 A), but nanoparticies have already been used in bio-medical applications for destroying metastases of tumors by short local overheating of the body tissue (US 2004219130 A).
The use of additives in shape memory polymers has until now mostly been the subject off studies that mainly focused on changes in the physical properties, for example the conductivity, the structure or the restoring forces during the switching process (Li F. et al.: Polyurethane/Conducting Carbon Black Composites: Structure, Electric Conductivity, Strain Recovery Behavior, and Their Relationships. Journal of Applied Polymer Science 75 (1), 68-77 (2000)). The type of the additives that were until now added in these studies to the shape memory polymers, range from carbon fibers (Gall K. et al.: Carbon fiber reinforced shape memory polymer composites. Journal of Intelligent Material Systems and Structures 11 (11), 877-886 (2001); Liang C. et al.: Investigation of shape memory polymers and their hybrid composites. Journal of Intelligent Material Systems and Structures 8 (4), 380-386 (1997)) and SiC (Gall K. et al.: Shape memory polymer nanocomposites. Acta Materialia 50, 5115-5126 (2002)) to metals (Gotthardt R. et al.: Smart materials based on shape memory alloys (SMAs): examples from Europe. Materials Science Forum 327-328 (Shape Memory Materials), 83-90 (2000); Monkmann G. J.: Advances in Shape Memory Polymer Actuation. Mechatronics 10, 489-498 (2000)). For example, the addition of carbon fibers to shape memory polymers is mainly intended to increase the strength or stiffness. Typical effects caused by the addition of metals or metal alloys as particle additives to the shape memory polymers are discussed in Monkmann (see above) based on comparative studies with a particle-free shape memory polymers and in Gotthardt R. in relation to damping oscillations in skis.
The company CTD (Lafayette, Colo., USA), which designed their product “Tembo” for application in space, describes the use of thermal conductors embedded in shape memory polymers. The shape memory function was here intended to be used for heating a hinge to unfold solar panels, which are still folded up when transported into space.
Iron oxide embedded in a silicate layer which heats up when an alternating magnetic field is applied has recently been offered by the company “Advanced Nanomaterials”.
A new approach is admixture of ferromagnetic or metallic particles to a shape memory polymer (US 20050212630 A). The particles are hereby used as “antenna” for activating the shape memory effect through an external stimulus. DE 10 2007 061 342.5 discloses corresponding mixtures of shape memory polymers with suitable particles, as well as a method for their production and thermal programming and switching. The content disclosed in DE 10 2007 061 342.5 is fully incorporated into the present application. The external stimulus is here an alternating electromagnetic field which generates heat in the ferromagnetic or metallic particles, causing a temperature increase of the shape memory polymer and hence activation of the shape memory effect. The magnitude of the attainable temperature increase depends on many parameters. These parameters include, on one hand, parameters determined by the EMF, in particular frequency and field strength of the alternating field. On the other hand, the maximally attainable material temperature for a predetermined field depends on the composition of the mixture, which contains a shape memory polymer and suitable nanoparticles, in particular on the size, the type, the quantity and the thermal conductivity of the nanoparticles. It has also been observed that the surface-to-volume ratio (O/V) also has an affect on the attainable material temperature due to the associated heat transfer to the surroundings. Therefore, shape memory bodies with a relatively small O/V can reach higher material temperatures in the EMF in than those with a large surface.
However, optimization of the aforementioned parameter is not always sufficient to attain the required switching temperature in the shape memory body. In the medical field, when for example a shape memory body is used as a stitching material in the human body, frequency and field strength of the EMF cannot be freely selected for the purpose of coupling higher energies into the shape memory body, because the body would otherwise be damaged. The concentration of the nanoparticles can be increased only in a limited way while substantially maintaining the mechanical properties of the shape memory body. Lastly, the surface-to-volume ratio also has application-related limits.
It is therefore an object of the present invention to provide a shape memory body which can be switched in an EMF, whose stored shape can be recalled even in an environment composed of materials with high thermal transfer coefficients.